The invention relates to a method and apparatus for calibrating a gas analyzer and, more particularly, the calibration of an automotive engine, exhaust analyzer.
Heretofore, a non-automatic calibration of an automotive exhaust analyzer required the operator to adjust two controls. One control varies a resistive potentiometer for zeroing the system display as the operator passes air through the analyzer. The second control also varies a potentiometer for adjusting system gain as the operator actuates a mechanical or electrical span reference to simulate a calibration gas passing through the analyzer.
The use of a span reference to simulate a calibration gas avoids the necessity of having an actual reference gas on hand for adjusting system gain. The span reference itself is variable by a third potentiometer for setting the value of the gas which the span reference is to simulate. The third potentiometer is set at the factory during manufacture of the analyzer; however, in certain cases, the span reference potentiometer may be accessible to the operator who is able to feed a known calibration gas through the analyzer for calibrating the span reference.
Attempting to make the calibration procedure easier for the operator and more reliable, an automatic calibration technique has been suggested wherein the final output of the analyzer is altered in view of calibration adjustment data formulated by the system. The operator actuates an instruction button for commanding the system to update the adjustment data. The automatic calibration technique utilizes a span reference instead of actual calibration gas, and requires three potentiometers for calibration of the span reference at the factory or by the operator who has access to actual calibration gas.
During calibration of the span reference, the most accurate calibration method occurs under a normal dynamic flow condition of the calibration gas through the analyzer. However, a large quantity of calibration gas is consumed during the time the operator is making the proper adjustment to the span reference.
To overcome consumption of the sample gas, the prior art has suggested a static flow condition, wherein the gas is injected into the analyzer and held statically allowing the operator to take his time in making the adjustment. However, this introduces calibration inaccuracies into the system due to drops in pressure and, hence, inaccuracies in the density of the gas in the analyzer with respect to the actual occurence of normal dynamic operating conditions.
It is, therefore, an object of the present invention to provide an improved calibration system for a gas analyzer.
It is another object of the present invention to provide a calibration system which includes a span reference and requires only a single operator adjustment for affecting calibration.
It is a further object of the present invention to provide a calibration system permitting consumption of a smaller amount of calibration gas during system calibration than, heretofore, realized.
It is yet another object of the present invention to permit adjustment to an analyzed reading after the gas has been removed from the analyzer cell.